Line an 8x8 baking pan with plastic wrap, extending the sheets of plastic over the sides. Set the ice cream out to soften.
Place the Oreos® in a large plastic zip-top bag and crush into large pieces with a rolling pin or meat mallet. Press half of the crushed cookies into the pan and freeze until firm. When the ice cream is soft, stir in the crushed candies and extract, if desired. Carefully spread softened ice cream onto the layer of crushed cookies and freeze for 30 minutes or until firm.
Warm the hot fudge topping in a microwave for 45 seconds and carefully spread over the chilled ice cream layer. Press the remaining crushed cookies gently into the fudge to secure, then freeze for 8 hours or until firm.
To serve, lift the bars out of the pan using the overhanging plastic wrap and transfer to a cutting board. Carefully peel the plastic away and cut into 9 bars. Serve immediately or wrap individual bars in plastic and freeze.